Measurement of the production cross section of three isolated photons in pp collisions at s=8 TeV using the ATLAS detector

Abstract

Measurement of the production cross section of three isolated photons in p p collisions at √ s = 8 TeV using the ATLAS detectorThe ATLAS Collaboration A measurement of the production of three isolated photons in proton-proton collisions at a centre-of-mass energy √ s = 8 TeV is reported. The results are based on an integrated luminosity of 20.2 fb −1 collected with the ATLAS detector at the LHC. The differential cross sections are measured as functions of the transverse energy of each photon, the difference in azimuthal angle and in pseudorapidity between pairs of photons, the invariant mass of pairs of photons, and the invariant mass of the triphoton system. A measurement of the inclusive fiducial cross section is also reported. Next-to-leading-order perturbative QCD predictions are compared to the cross-section measurements. The predictions underestimate the measurement of the inclusive fiducial cross section and the differential measurements at low photon transverse energies and invariant masses. They provide adequate descriptions of the measurements at high values of the photon transverse energies, invariant mass of pairs of photons, and invariant mass of the triphoton system.The production of three prompt photons in proton-proton (pp) collisions, pp → γγγ + X, provides a testing ground for perturbative quantum chromodynamics (pQCD). This process is rare in the Standard Model (SM) since the leading-order (LO) contribution to triphoton production is of order α 3 EM . The measurement of triphoton production can be performed in a broader range of kinematic regions than in 2 → 2 reactions such as inclusive-photon [1-4] and diphoton [5][6][7] production. This provides a complementary test of pQCD in processes with photons in the final state.Precise measurements of triphoton production can be used to improve the description of this process in Monte Carlo (MC) models. In addition, SM triphoton production provides one of the main irreducible backgrounds for some beyond-the-SM (BSM) searches. Potential BSM processes include the associated production of a photon and an exotic neutral particle decaying into a photon pair (qq → X 0 γ), where X 0 can be a Kaluza-Klein graviton (G KK ) [8][9][10] or a pseudoscalar (a) [11]. Moreover, triphoton production is also the main background to the predicted decay of the Z boson into three photons. The current upper limit at 95% confidence level on the branching fraction for Z → 3γ is 2.2 × 10 −6 [12].Three photons can be produced via two main mechanisms: direct and fragmentation production. In the case of the direct production process, three photons are produced in the hard interaction via the annihilation of an initial-state quark-antiquark pair (qq → γγγ). In the fragmentation process, at least one of the photons arises from the fragmentation of a high-transverse-momentum (high-p T ) parton (qg → γγq[γ]). Direct photons are typically isolated, while those originating from the fragmentation process are usually accompanied by nearby partons. Measurements of final-state photons incl...

Leading-color two-loop QCD corrections for three-photon production at hadron colliders

“…Similar to the related two-photon production process [9][10][11][12][13][14], threephoton production exhibits very slow perturbative convergence [15,16] and non-reliable estimates of uncertainty from missing higher orders. In particular, a significant tension between LHC Run 1 data and NLO QCD predictions has been reported [17]. The computation of NNLO QCD corrections is thus crucial for this process.…”

Pentagon functions for scattering of five massless particles

“…The target values are obtained with the package PentagonFunctionsM, see section 7.1 9. The subset of the pentagon functions contributing to master integrals of the pentagon and the planar pentagon-box topologies.10 More concretely, we use an integration grid, optimized for Monte-Carlo integration of the leading order qq → γγγ matrix elements over the fiducial phase space defined by the analysis of[85]. We used MATRIX[86] to obtain the integration grid.…”

Gravitational waves from first-order phase transition in a simple axion-like particle model

“…Similarly, there are also searches of radiative decays of Upsilon mesons Υ → γ + / E T at CLEO [159] and BaBar [160], which can be used to set limits on the ALP coupling g aγγ . If the ALP decays promptly in the detectors, then we have the signatures of three photons e + e − , pp, pp → γ + a → γγγ at LEP [161,162], Tevatron [163] and LHC [164,165]. These limits could be improved by one to two orders of magnitude at future colliders such as Belle II [166], ILC [167], FCC-ee [168] and later stages of LHC [169,170].…”